Technical Field
The present disclosure relates to solutions or dispersions of carbon nanotubes (CNT) and methods for producing the solutions or dispersions. More particularly, the present disclosure relates to solutions or dispersions of carbon nanotubes with high concentration and low contaminations, and methods for purifying the CNT solution or dispersions.
Related Art
Carbon nanotubes (CNT) are useful for many applications. At the present time, engineers have been successful in building semiconductor devices from CNTs by taking advantage of the conducting and/or semiconducting properties of the CNTs. For example, individual nanotubes may be used as conducting elements, e.g. as the channel of a transistor. However, the placement of millions of catalyst particles and the growth of millions of properly aligned nanotubes of specific length presents serious challenges. U.S. Pat. Nos. 6,643,165 and 6,574,130 describe electromechanical switches using flexible nanotube-based fabrics (nanofabrics) derived from solution-phase coatings of nanotubes in which the nanotubes first are grown, then brought into solution, and applied to substrates at ambient temperatures. Nanotubes may be derivatized in order to facilitate bringing the tubes into solution. However, in uses where pristine nanotubes are necessary, it is often difficult to remove the derivatizing agent. Even when removal of the derivatizing agent is not difficult, such removal is an added, time-consuming step.
Generally, solvents being used to solubilize and disperse the carbon nanotubes are organic, such as ODCB, chloroform, ethyl lactate, to name just a few. The solutions are stable but the solvents have the disadvantage of not solubilizing clean carbon nanotubes which are free from amorphous carbon. A method has been developed to remove most of the amorphous carbon and solubilize the carbon nanotubes at high concentrations in water via pH manipulation.
There have been few attempts to disperse SWNTs in aqueous and non-aqueous solvents. Chen et al. first reported solubilization of shortened, end-functionalized single-walled nanotubes (SWNTs) in solvents such as chloroform, dichloromethane, orthodichlorobenzene (ODCB), CS2, dimethyl formamide (DMF) and tetrahydrofuran (THF). See, “Solution Properties of Single-Walled Nanotubes,” Science 1998, 282, 95-98. Ausman et al. reported the use of SWNTs solutions using sonication. The solvents used were N-methylpyrrolidone (NMP), DMF, hexamethylphosphoramide, cyclopentanone, tetramethylene sulfoxide and ε-caprolactone (listed in decreasing order of carbon nanotube solvation). Ausman at el. generally conclude that solvents with good Lewis basicity (i.e., availability of a free electron pair without hydrogen donors) are good solvents for SWNTs. See, “Organic Solvent Dispersions of Single-Walled Carbon Nanotubes: Toward Solutions of Pristine Nanotubes,” J. Phys. Chem. B 2000, 104, 8911. Other early approaches involved the fluorination or sidewall covalent derivatization of SWNTs with aliphatic and aromatic moieties to improve nanotube solubility. See, e.g., E. T. Mickelson et al., “Solvation of Fluorinated Single-Wall Carbon Nanotubes in Alcohol Solvents,” J. Phys. Chem. B 1999, 103, 4318-4322.
Full-length soluble SWNTs can be prepared by ionic functionalization of the SWNT ends dissolved in THF and DMF. See, Chen et al., “Dissolution of Full-Length Single-Walled Carbon Nanotubes,” J. Phys. Chem. B 2001, 105, 2525-2528 and J. L. Bahr et al Chem. Comm. 2001, 193-194. Chen et al. used HiPCO™ as-prepared (AP)-SWNTs and studied a wide range of solvents. (HiPCO™ is a trademark of Rice University for SWNTs prepared under high pressure carbon monoxide decomposition). The solutions were made using sonication.
Bahr et al. (“Dissolution Of Small Diameter Single-Wall Carbon Nanotubes In Organic Solvents,” Chem. Comm., 2001, 193-194) reported the most favorable solvation results using ODCB, followed by chloroform, methylnaphthalene, bromomethylnaphthalene, NMP and DMF as solvents. Subsequent work has shown that good solubility of AP-SWNT in ODCB is due to sonication induced polymerization of ODCB, which then wraps around SWNTs, essentially producing soluble polymer wrapped (PW)-SWNTs. See Niyogi et al., “Ultrasonic Dispersions of Single-Walled Carbon Nanotubes,” J. Phys. Chem. B 2003, 107, 8799-8804. Polymer wrapping usually affects sheet resistance of the SWNT network and may not be appropriate for electronic applications where low sheet resistance is desired. See, e.g., A. Star et al., “Preparation and Properties of Polymer-Wrapped Single-Walled Carbon Nanotubes,” Angew. Chem. Int. Ed. 2001, 40, 1721-1725 and M. J. O'Connell et al., “Reversible Water-Solubilization Of Single-Walled Carbon Nanotubes By Polymer Wrapping,” Chem. Phys. Lett. 2001, 342, 265-271.
While these approaches were successful in solubilizing the SWNTs in a variety of organic solvents to practically relevant levels, all such attempts resulted in the depletion of the electrons that are essential to retain interesting electrical and optical properties of nanotubes. Other earlier attempts involve the use of cationic, anionic, or non-ionic surfactants to disperse the SWNT in aqueous and non-aqueous systems. See, Matarredona et al., “Dispersion of Single-Walled Carbon Nanotubes in Aqueous Solutions of the Anionic Surfactant,” J. Phys. Chem. B 2003, 107, 13357-13367. While this type of approach has helped to retain the electrical conductivity and optical properties of the SWNTs, most such methods leave halogens or alkali metals or polymeric residues, which tend to severely hamper any meaningful use in microelectronic fabrication facilities.
There is a need for a method of solvating or dispensing nanotubes in solvents for use in electronics applications. Such a method can allow for removal of amorphous carbon and other contaminants, leaving carbon nanotubes a high concentration of CNTs in solution. Such a solution could be useful for making high-uniformity nanotube fabrics on various substrates including silicon. The use of such a solution would require few applications (i.e. spin coat applications), to produce a fabric of controllable sheet resistance with high uniformity. Such a solution could have many other applications as well. There is a further need for methods that meet the criteria outlined above for low toxicity, purity, cleanliness, ease of handling, and scalability.